1. Field of the Invention
The invention relates to control systems and more specifically to Quasi-resonant DC link converters for use in conjunction with AC machines.
2. Descriptions of the Prior Art
Resonant link DC to DC converters utilizing zero voltage or zero current switching schemes are available in the prior art. The resonant link concept is being extended to DC to AC converters for AC machine applications. Resonant link converters can be divided into two classes, the first using shunt resonant circuits and the second series resonant circuits. In applications such as controlling AC motors the output of the converter is coupled to the motor through a conventional inverter.
Conventional inverters may utilize either zero voltage or zero current voltage switching to reduce power loss. Zero voltage switching is currently preferred because it reduces the voltage stress on switching devices and does not require the use of snubber circuits.
Voltage overshoot and the resulting stress of switching components is a generic problem with resonant link converters.
Typically, if the resonant converter operates without any load, the converter output voltage swings to twice the supply voltage. As load changes, the voltage swing varies up and down. In situations where bi-directional energy transfer is provided for, the output voltage of the converter can swing to three times the supply voltage. One prior art approach to reducing this voltage swing is the utilization of voltage clamping with energy recovery. Additionally, problems are encountered during transition from the utilization to the energy feed-back mode.
Reliable operation requires that the output voltage of the converter cross zero at every cycle. In prior art resonant link converters, the output voltage frequently failed to cross zero due to a finite "Q" of the resonant circuit, resulting in the voltage tending to build up on every resonant cycle.
It has been proposed that a fixed initial current be established in the inductor of the resonant link to solve the zero crossing problem. However, fluctuating and directional inverter input current tends to cause zero crossing failure unless the initial current is established on a worse case basis. Establishing an initial current on a worse case basis, worsens the voltage overshoot problem. Controlling the initial current in accordance with the criteria disclosed in this patent application solves the problems associated with a fixed initial current.
In some applications it is also desirable to vary the frequency of the converter. Varying the frequency can be utilized to reduce the circulating phase currents. Reducing the circulating phase currents reduces electrical losses, reduces electrical noise and reduces mechanical noise. Prior to the current invention, no solution to this problem has been proposed or implemented.